Ultrasound generally refers to sound waves that have a frequency or pitch above the range of human hearing. In an ultrasonic imaging system, typically, short bursts of acoustic energy are directed into a body and the reflective energy or echoes are received at a later time. The amplitude of the reflective energy is processed and formatted into a video image of the insonified body. Ultrasonic imaging is used widely in medical applications to non-invasively "see" inside the human body, particularly the cardiac structures, the vascular system, the fetus, the uterus, the abdominal organs and the eye.
One application where ultrasonic imaging is particularly useful is echocardiography or imaging of the heart, particularly the myocardium. In echocardiography, ultrasound is used to image the myocardium and identify any irregularities in its structure. A known echocardiography technique is referred to as myocardial perfusion echocardiography. In this technique, a contrast agent is introduced into the coronary arteries. The dissipation or wash-out of the contrast agent from the myocardium is monitored with ultrasonic imagery. The rate at which the contrast agent dissipates from a region of the myocardium provides an indirect measurement of the blood flow or perfusion characteristics of that region of the myocardium. Such data is useful in determining whether or not a portion of the coronary arteries is blocked by a stenotic lesion. The technique is outlined in more detail in an article entitled "Myocardial Perfusion and Contrast Echocardiography: Review and New Perspectives", Vandenberg, Echocardiography: A Journal of CV Ultrasound and Allied Technologies, Vol. 8, No. 1, 1991.
One contrast agent which has been used successfully in myocardial perfusion techniques is microbubbles. Such microbubbles are approximately 4-7 microns in size, approximately the size of red blood cells, and are released as a bolus into one of the coronary arteries, typically through a percutaneously inserted angioplasty catheter. Since air pockets are highly reflective of ultrasonic energy, the presence of microbubbles within the myocardium causes a high reflection of acoustic energy resulting in a visually discernable area of brightness or intensity in the ultrasonic video image. The region of the myocardium with the greatest microbubble concentration appears as a region of high intensity on a grayscale ultrasonic image. This intensity decreases at a rate dependent upon the rate of perfusion through the myocardial tissue.
However, the relationship between microbubble concentration and the gray level intensity of the ultrasonic image is non-linear and dependent on the settings of the various imaging controls. Therefore, the video image is an inaccurate representation of the true myocardial perfusion characteristics. This non-linearity is due, in part, to the compression of the ultrasonic signals, which may have a dynamic range greater than 100 dB after amplification, to a smaller dynamic range, typically less than 20 dB, which is required so that the signals can be displayed on commercially available grayscale imaging equipment. In addition, preprocessing adjustments alter the relationship between the analog signal and the gray scale value of the image. Further, post-processing adjustments may vary the relationship between the digitized gray scale level and the displayed output gray levels in the imaging system.
In an attempt to overcome some of the above problems, systems have been developed in which the average intensity for the selected ultrasonic video data is corrected for background offset before the time-intensity curve of such data is plotted, as shown by the solid line in FIG. 1. Such a system is described in an article entitled "Workshop on Contrast Echocardiography: Myocardial Perfusion", Feinstein, Echocardiography: A Journal of CV Ultrasound and Allied Technology, Vol. 6, No. 4, 1989. The resulting time-intensity curve is then smoothed with a computer program, as shown by the dotted line in FIG. 1. However, the disclosed system has a number of drawbacks.
First, the process of extracting intensity data from stored video images and generating a time-intensity curve typically takes several hours and occurs "off-line" once the echocardiographic procedure, and any accompanying revascularization procedure has terminated. If the time-intensity plot is being used to determine tissue perfusion characteristics and, ultimately, the success of an angioplasty procedure, the results will not be available until the procedure has ended and the patient is no longer immediately available. Accordingly, a need exists for a way to acquire intensity data and generate a time-intensity plot in real-time while the echocardiography procedure is in progress and before any accompanying angioplasty or revascularization procedures have terminated.
Second, commercially available ultrasonic imaging systems offer various levels of both compression and postprocess gray scaling. Even careful analysis of stored video data cannot indicate what compression level and gray scale mapping were used when the video image was created. Compensation for the background intensity of the video image will not correct the non-linearities introduced into the video data by the compression and scaling of the ultrasonic signals. Also, smoothing of the time-intensity curve, as in the above-referenced article, will not compensate for such non-linearities.
Accordingly, a need exists for a way to reverse the non-linearities introduced by the compression or postprocess gray scaling of ultrasonic data so that an accurate time-intensity curve may be generated from a real-time or stored ultrasonic image, such curve having a direct linear relationship with the concentration of contrast agent in the selected region of the scanned tissue.
It is, therefore, an object of the present invention to provide a system in which the non-linearities introduced by compression and gray scaling of ultrasonic data into a video image may be reversed.
A further object of the present invention is to provide a system in which non-linearities caused by compression and gray scaling of ultrasonic data into video images may be reversed whether the video images are taken from a stored frame memory or real time buffer memory.
Yet another object of the present invention is to provide a system which can calculate the average intensity of a specified region of an ultrasonic video image in which the average intensity is linearly related to the received echo signals.
Still a further object of the present invention is to provide a system which can generate a time-intensity plot of ultrasonic data which accurately reflects the myocardial perfusion characteristics during a contrast echocardiography procedure.
A further object of the present invention is to provide a tool for quantitative measurement and display of the average acoustic signal in a user specified region of a two dimensional ultrasonic image.